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The foodborne pathogen, Eschericia coli O157:H7, causes about 73,000 cases of illness/year in the U.S. Life-threatening complications include hemorrhagic colitis and hemolytic uremic syndrome. A major disease determinant for O157:H7 is the production of Shiga toxin (Stx) in the intestine. Two immunologically distinct types of Stx, Stx1 and Stx2, can be produced by O157:H7, and the production of Stx2 has been epidemiologically linked to more severe disease. Regulation of the Shiga toxins by E. coli is unique. The toxin genes are encoded in the late gene region of a lambdoid bacteriophage lysogenized in the O157:H7 chromosome. The toxin genes are silent during lysogeny, and induction of the phage to enter the lytic cycle results in the production of phage and, concomitantly, toxin. We examined the hypothesis that during infection, toxin-encoding phage produced by O157:H7 infect intestinal E. coli , resulting in amplification of phage and toxin production. We have shown that this phenomenon does occur, both in vitro and in vivo. Clinical O157:H7 strains were further studied. Stx2-encoding phage from seven O157:H7 isolates were found to be variable with respect to protein profiles, cross-immunity and cross-lysogeny, even when the phage originated from O157:H7 isolates that were indistinguishable by molecular typing. We also studied the interaction of these phage with normal human E. coli . Only two of the seven phage were able to amplify toxin in normal E. coli , indicating that this phenomenon, while present, is rare. Lysogeny of normal E. coli by these two phage differed, and further inspection of the results suggests that normal E. coli that belong to the phylogenetic groups A1 and B are more susceptible to lysogeny. Characterization of the interaction between Stx phage and normal E. coli lead to the discovery of FI-29, a normal E. coli isolate that neutralizes Stx2, but not Stx1, by binding the toxin. Lipopolysaccharide (LPS) was shown to be the neutralizing agent. FI-29 LPS belongs to the O107/O117 serotypes, and E. coli strains of either serotype also neutralize toxin. FI-29 may be a potential probiotic or therapeutic for the prevention and treatment of O157:H7 disease.